1. Field of Invention
This invention relates to a method and an apparatus for transferring a substrate, particularly a method and an apparatus for transferring a wafer between hot plates.
2. Description of Related Art
Hot plates are widely used to heat solids or liquids. For a thermal process that includes two or more stages of different temperatures, usually the same number of hot plates are used to achieve continuous processing of multiple substrates. For example, a photoresist reflow process usually includes two heating steps at different temperatures and uses two hot plates for the two heating steps respectively.
FIG. 1 shows the temperature profile for such a photoresist reflow process as a two-step thermal process in the prior art. The process may be performed to a patterned photoresist layer 110 that is formed on a substrate 100 and has therein an opening 120, so as to reduce the size of the opening 120. The reflow process includes a first heating step at a first temperature T1 between the time points t1 and t2 and a second heating step at a second temperature T2 (>T1) between t3 and t4, with a transfer time for transferring the substrate from the first hot plate to the second one between the two heating steps. The resulting opening 120a after the second heating step has a smaller size.
Conventionally, a substrate is moved between hot plates by a robot. However, since the movement by a robot takes a relatively long time of about 8-15 seconds, a significant temperature drop occurs during the transfer time so that the thermal dosage and the resulting opening shrinkage and critical dimension (CD) are difficult to control. In addition, the throughput is lowered due to the slow substrate transfer.